1. Field of Invention
The present invention relates to remote arc welding apparatus and more particularly to a simple control system having a two conductor control cable from the remote unit to the control circuitry in which good wire feed motor speed stability is obtained over a wide range of operating speeds.
2. Description of Prior Art
In remote welding systems it is desirable to minimize the number of leads between the central control and power unit and the remote unit containing the wire feeder. It is further desirable to minimize the chances that cables, once severed, will be reconnected incorrectly thus damaging the equipment.
Known art in remote welding systems has generally taken one of two approaches to the problem of controlling welding current, wire feed and shielding gas supplies at the workpiece. A remote welding system consists of multiple power supplies and remote units; one such supply and remote unit for each welder. The power supply generally has a 460 volt alternating current input and a high current DC output of about 25 volts. Between the power supply and the remote unit are the welding cable and various control cables. A major problem in remote welding systems is that in a large system such as that found at a shipyard, it is impossible to prevent a multitude of cables crisscrossing at various locations from being severed accidentally. Prior art remote welding systems have used systems to control the wire feed, weld current and shielding gas that have caused numerous problems in the presence of crossed cables and accidental cable severance.
The most common form of control in prior art welding systems uses 115 volt AC control voltages and cables with at least five conductors from the power supply to the remote unit in addition to the welding cable. When accidentally severed, these multi wire control cables expose workers to hazardous 115 volt AC voltages. Furthermore experience has taught that repair of such cables require specialized maintenance personnel and results in added expense, and large amounts of down time for the welder.
The remote units are usually 100 to 200 feet from the welding power supply and contain the wire feed motor. When remote systems using wire feed rather than stick electrodes were first introduced, the wire feed motor did not have good speed stability at the remote location. More recent systems have used silicon controlled rectifiers (SCR's) and other semiconductors to regulate the speed of the wire feed motor at the remote location. Upon accidental severance of the multiconductor control cables, short circuits in the 115 volt control system often destroy the regulating semiconductor devices causing additional expense and delay.
The multi conductor control cables of prior art remote welding systems are bulky and cumbersome for the welder to move since the welders are working one or two hundred feet from their power supplies.
Some prior art systems have overcome the problem of bulky control cables and 115 AC voltages by modulating radio frequency control signals on the welding cable. While this approach eliminates the multi conductor cable problem it creates or amplifies other problems associated with remote welding systems.
It should first be noted that a radio frequency modulation system requires sophistocated electronic components that are much more difficult to service than simpler devices such as relays. A modulation system requires some of these electronics to be located at the remote unit and the circuits are usually fabricated on printed circuit boards. Such printed circuit boards are very susceptible to damage from mechanical shocks that are often encountered at the remote locations. In a large system with many welding cables crisscrossing working areas modulation systems have been found to be susceptible to cross talk of the modulated signals. Such systems usually have a discrete frequency for each welder and his supply and in practice it has been found that spurious harmonic and intermodulation products appear on various cables thus causing some of the control circuitry to be turned on and off when the welder using it has given no such signal. When SCR control for wire feed motors are used with radio frequency modulation systems, the high noise generated by the SCRs produces further spurious responses. As is known to those skilled in the art, radio frequency modulation systems are also susceptible to spurious signals that are produced by transients that are present in a large welding system.